1. Field of the Invention
The present invention relates to a method for foam-injection molding a thermoplastic resin. More particularly, the present invention is concerned with a method for foam-injection molding a thermoplastic resin to produce a foamed molded article having a substantially non-foamed, surface skin layer, wherein the method comprises: (1) providing a mold comprising a fixed mold half and a movable mold half mating with the fixed mold half to thereby form a mold cavity defined by an inner wall of the fixed mold half and an inner wall of the movable mold half, wherein the mold cavity has an inner wall which has a molten resin releasing means, (2) injecting a foamable thermoplastic resin in a molten form into the mold cavity of the mold, thereby forming a foamable molten resin mass in the mold cavity, (3) applying a pressure to the foamable molten resin mass in the mold cavity to press the surface of the foamable molten resin mass against the inner wall of the mold cavity, thereby allowing a surface portion of the foamable molten resin mass to be solidified to form a surface skin layer of the foamable molten resin mass, and (4) releasing a portion of the foamable molten resin mass under the foaming pressure exerted by the foamable molten resin mass to the outside of the mold cavity through the molten resin releasing means, thereby lowering the pressure exerted on the foamable molten resin mass, so that foaming of the foamable molten resin mass in the mold cavity is caused to occur to form a foamed resin mass having the surface skin layer remaining substantially non-foamed. The foam-injection molding method of the present invention is advantageous not only in that a molded article which exhibits excellent reproduction of the morphology of the inner wall of the mold cavity and which has both a non-foamed surface skin layer and a highly foamed interior portion can be produced with excellent reproducibility and high efficiency and economically, but also in that the thickness of the surface skin layer and the expansion ratio of the molded article can be easily controlled. The foam-injection molding method of the present invention can provide various excellent foam-injection molded articles of a thermoplastic resin at a low cost. Examples of such molded articles include a housing for light electrical equipment or electronic equipment, various automobile parts, and every day commodities. Further, the foam-injection molding method of the present invention can be advantageously used not only for molding ordinary thermoplastic resins, but also for molding various resin compositions which contain a flame retardant having a low thermal stability and hence which cannot be molded at high molding temperatures, and for molding various resins having such a low fluidity that it is difficult to mold the resins by a conventional injection molding method.
2. Prior Art
For the production of a foamed molded article, conventional foam-injection molding methods are known which include a step of foaming a foamable thermoplastic resin containing a foaming agent. In the conventional foam-injection molding methods, azodicarbonamide (ADCA), N,N′-dinitrosopentamethylenetetramine (DPT) or the like is generally used as the foaming agent in an amount in the range of from 1 to 5 parts by weight, relative to 100 parts by weight of the resin.
As a representative example of the above-mentioned conventional foam-injection molding methods, a method called “short shot method” can be mentioned. In this method, a molten resin containing a foaming agent is injected into a mold cavity in an amount corresponding to a volume which is less than the volume of the mold cavity. When a molten resin is injected into a mold cavity, a forward portion of the injected molten resin, which first contacts with the inner wall of the mold cavity, is quickly cooled to thereby form a solidified layer. Then, a backward portion of the injected molten resin flows through the central portion of the mold cavity along the solidified layer, then reaches and passes through the flow front of the solidified layer, and then goes toward and finally contacts with the inner wall of the mold cavity, where the molten resin is cooled to thereby form a new solidified layer. Such flow behavior of the injected molten resin in the mold cavity is called a “fountain flow”. After the injection, the molten resin in the mold cavity foams and expands until it reaches the inner periphery of the mold cavity to thereby fill up the mold cavity and form a foamed molded article. This so-called “short shot method” is a simplest form of foam-injection molding method and is widely employed, although this method poses a problem in that the surface of the obtained molded article becomes rough due to the occurrence of foaming at the flow front of the injected molten resin.
On the other hand, as a method for obtaining a large-thickness, foamed molded article having excellent appearance and exhibiting less occurrence of a sink mark or warping, there is known a conventional foam-injection molding method which is generally called a “counter pressure method”. An example of a counter pressure method is described in Examined Japanese Patent Application Publication No. Sho 62-16166. The counter pressure method comprises a step of injecting a molten resin containing a foaming agent into a mold cavity filled with a compressed air, wherein the amount of the injected molten resin is a “short shot”, relative to the volume of the mold cavity; a step of releasing the compressed air from the mold cavity to the outside of the mold; and a step of cooling the molten resin in the mold cavity while keeping the molten resin under a low pressure. By this method, the flow front of the injected molten resin can be prevented from foaming, so that there can be obtained a foamed molded article in which the surface is free of a foam mark and only the interior portion is foamed. In the counter pressure method, a foamable molten resin is injected into a mold cavity so that the mold cavity is almost filled with the injected foamable molten resin in non-foamed condition, and then, the injected molten resin is cooled and hence shrinks while exhibiting foaming expansion to compensate for the volume decrease due to the shrinkage. In this method, basically, the foaming agent is used in a minimum amount which can cause a foaming expansion sufficient for compensating for the volume decrease due to the shrinkage. Therefore, this method has a problem in that, since the molten resin cannot be pressed against the inner wall of the mold cavity under a high pressure, the obtained molded article exhibits poor reproduction of the morphology of the inner wall of the mold cavity, and a poor appearance. This method has also a problem in that the foaming expansion is caused only to such an extent as is sufficient for compensating for the volume decrease due to the shrinkage, a great reduction in the weight of the molded article cannot be obtained. Further, there is also a problem in that, when the foaming agent is used in too large an amount, the obtained molded article continues foaming even after the article is taken out from the mold cavity, thus causing an expansion or distortion of the obtained molded article. For preventing this problem, it is necessary that the cooling time (i.e., the time for which the injected molten resin is left to cool in the mold cavity) be extremely long.
A further example of the above-mentioned conventional foam-injection molding methods is as follows. An old method is known which comprises a step of injecting a foamable molten resin into a mold cavity, a step of introducing a gas into the injected molten resin to thereby form a hollow mass of molten resin, a step of allowing the hollow mass to stand until its outer surface portion is solidified, and a step of releasing the introduced gas to the outside of the mold, thereby causing the inner portion of the hollow mass to foam and expand inwardly to fill the hollow (see Examined Japanese Patent Application Publication No. Sho 53-25352 (corresponding to U.S. Pat. No. 4,129,635)). This method has the following disadvantages. The introduced gas (which was used for forming the hollow) is released through the gas outlet (which is the same as the gas inlet through which the gas is introduced); however, due to a great pressure loss inside pipes or the like connected to the gas outlet (inlet), it takes a long time to release the introduced gas through the gas outlet (inlet). Hence, the solidification of the inner portion of the hollow mass of molten resin progresses to a great extent before the gas has been completely released, resulting in that the molten resin cannot form a foamed interior portion having a satisfactory size. Further, in the case where the foamable molten resin has a high content of a foaming agent and hence has a high foamability, when the inner portion of the hollow mass of molten resin is caused to foam by the release of the gas through the gas outlet (inlet), foaming occurs also at a portion of the molten resin around the gas outlet (inlet), leading to a clogging of the gas outlet by the foam. As a result, the gas used for forming the hollow cannot be smoothly and completely released to the outside of the mold, leading to a problem in that an internal gas pressure remains in the hollow even at the end of the cooling time, and the remaining internal pressure will cause the obtained molded article to burst when the mold is opened for taking-out of the molded article from the mold cavity.
Also, there have recently been proposed methods which are similar to the above-mentioned foam-injection molding method in which a hollow mass of a foamable molten resin is first produced and then foaming of the hollow mass is caused to occur. For example, such methods are described in Unexamined Japanese Patent Application Laid-Open Specification No. Hei 7-32405 (corresponding to U.S. Pat. Nos. 5,900,198 and 5,948,446) and Unexamined Japanese Patent Application Laid-Open Specification No. 2000-94468. Disclosed in the former (i.e., Unexamined Japanese Patent Application Laid-Open Specification No. Hei 7-32405) is a method for producing an injection-molded hollow resin article which has a hollow in a large thickness portion thereof and which has gas cells interspersed between the surface skin layer and the hollow portion, wherein the resin article exhibits less occurrence of a sink mark in the surface thereof. In this method, a foamable molten resin is injected into a mold cavity which is pressurized with a first high pressure gas, and a second high pressure gas is introduced into the injected molten resin during or after the injection of the molten resin, to thereby form a hollow in the injected molten resin. Then, the high pressure gas in the mold cavity is released to the outside of the mold, to thereby cause the foamable molten resin to foam and expand inwardly into the hollow. As in the case of the above-mentioned method disclosed in Examined Japanese Patent Application Publication No. Sho 53-25352 (corresponding to U.S. Pat. No. 4,129,635), this method has problems not only in that it takes a long time to release the gas used for forming the hollow, thus making it impossible to form a foamed portion having a satisfactory size, but also in that occurrence of foaming around the gas outlet causes a clogging of the gas outlet and hence hinders the release of the gas, leading to an occurrence of bursting of the obtained molded article due to the pressure of a residual gas.
On the other hand, the latter of the above-mentioned two documents (i.e., Unexamined Japanese Patent Application Laid-Open Specification No. 2000-94468) discloses a method for producing a lightweight molded article which has a substantially non-foamed surface skin layer and a foamed interior portion and which exhibits excellent appearance, high strength and high stiffness. In this method, a molten resin containing a foaming agent is injected into a mold cavity so as to fill the mold cavity, and a pressurized gas is introduced into the injected molten resin so as to release a portion of the injected molten resin into a resin release cavity (which is called a “spillover cavity”), to thereby form a hollow, followed by releasing of the pressurized gas in the hollow to lower the gas pressure. This method has a defect in that, even when a portion of the injected molten resin is released into the resin release cavity, the gas in the hollow remains under a high pressure; as a result, although the gas in the hollow is released thereafter, substantially the same problems as in the case of the other (described above) of the two documents are posed (namely, the problems that the injected molten resin cannot form a foamed portion having a satisfactory size, and bursting of the obtained molded article is caused by the pressure of a residual gas).
As apparent from the above, there has not yet been known a foam-injection molding method which is advantageous in that a high expansion ratio of the obtained molded article can be obtained to thereby achieve a decrease in weight of the molded article and a decrease in the time of molding cycle, while maintaining excellent appearance and high dimensional accuracy of the molded article.
Further, with respect to the case of a molding operation in which a plurality of molded articles are obtained in one molding cycle, the conventional foam-injection molding methods have the following disadvantages. In the case where it is intended to obtain two or more molded articles at a time by using a mold designed for such purpose (i.e., having a plurality of mold cavities for molding a plurality of articles at a time), when it is desired for the plurality of molded articles to have the same expansion ratio, it is necessary that, with respect to the molding conditions, such as the amount of resin injected into each mold cavity and the resin pressure, a good balance be achieved between the plurality of mold cavities of the mold. However, it is difficult to achieve such a good balance between the plurality of mold cavities of the mold. Especially, when four or more molded articles are produced at a time by the conventional molding methods, in most cases, it is impossible to achieve a good balance (with respect to the molding conditions) between the plurality of mold cavities of the mold, and hence it is almost impossible for the plurality of molded articles to have the same expansion ratio. Further, when a plurality of molded articles having varied volumes are produced at a time by using a single mold (having a plurality of mold cavities having varied volumes), it is even more difficult to achieve a good balance (with respect to the molding conditions) between the plurality of mold cavities of the mold. For these reasons, at present, production of a plurality of foam-injection molded articles simultaneously in one molding cycle is not employed in the large scale production of foam-injection molded articles.
An object of the present invention is to provide a method for producing a lightweight, foamed molded article which exhibits excellent reproduction of the morphology of the inner wall of the mold cavity and which has a high dimensional accuracy, at a high productivity and at a low cost.